The invention relates to single-piston, two-cycle gasoline engines and more particularly, techniques for eliminating certain prior art machining operations performed on cylinder head and crankcase castings.
Current manufacturing techniques involve casting a cylinder block and a crankcase using a die-casting process utilizing standard casting tolerances that are relatively broad. The cast cylinder and crankcase go through numerous machining steps to arrive at the finished product, ready to be assembled together, and with additional engine parts, into a completed engine.
Traditionally, a typical die casting process employs “standard casting tolerances”, which are known as “steel safe”. “Steel safe” means that the core pins that are used to produce holes in a part are on the high side of broad tolerances so that as wear occurs on them, they would nevertheless remain in tolerance. Die details that create the outside surface of the casting are dimensioned on the low side of the broad tolerance so that wear on the die allows the resultant part to remain in print tolerance. This allows a die to produce large quantities of parts with little attention paid to the dimensional integrity of the parts, resulting in a low maintenance cost.
At least in the manufacture of cylinder blocks and crankcases for single-piston, two-cycle gasoline engines, these savings are illusory in that mating surfaces, such as the mating surface between the block and the crankcase, must be machined. Also, the broad tolerance core pin openings must be drilled and tapped to receive the fasteners for these parts. Further, the crankshaft bearing portal must be machined to a press tolerance and machined to accommodate bearing locator snap rings. All of these machining operations require labor and equipment costs, which negate any savings in employing standard casting tolerances.
In addition to the cost factors involved in machining the foot area of the cylinder head and the mating area of the crankcase to ensure a proper seal, the machining operation itself contributes to exhaust gas leaks in the casting. All aluminum die castings are inherently porous. However, the initially chilled surface of the casting provides a dense skin, which seals the porous interior of the casting. When this skin is machined to provide precise gasket mating surfaces between the cylinder block and crankcase, the dense skin is removed and exhaust leakage is permitted through the gasket area.
Analyzing the costs of the traditional machining operations, including the costs of the machine tools, the labor involved in operating the machine tools, the time loss due to the number of steps involved, and the risks of poor quality due to potential errors that the large number of operations required can cause led to the realization that by requiring tighter tolerances on the die mold and its components, one could decrease the total cost of the manufacturing process despite the increased die mold and maintenance costs and the decreased die mold life.